The present invention pertains to a seat belt retractor for accommodating a seat belt to be retracted and withdrawn, and more particularly, to a torsion bar used in a seat belt retractor as a force limiter mechanism (hereinafter, sometimes referred to as EA mechanism) for limiting the load exerted on the seat belt.
Conventionally, a seat belt device installed in a vehicle, such as an automobile, prevents an occupant from being thrown out of a vehicle seat by restraining the occupant with the seat belt thereof, thereby protecting the occupant in the event of an emergency, such as vehicle collision, at which large deceleration is exerted on the vehicle.
Such a seat belt device is provided with a seat belt retractor for accommodating a seat belt to be retracted and withdrawn. The seat belt retractor has biasing means, such as a spiral spring, for always biasing a reel, around which the seat belt is wound, in a retracting direction. When it is not used, the seat belt is fully wound on the reel by the biasing force of the biasing means. When an occupant wears the seat belt, the seat belt is withdrawn against the biasing force of the biasing means. In the seat belt retractor, a locking mechanism is activated in the event of an emergency as mentioned above to stop rotation of the reel in an unwinding direction, thereby preventing the seat belt from being withdrawn. Therefore, the seat belt can securely restrain and thus protect the occupant.
However, the conventional seat belt retractor of the seat belt device has a problem. That is, when the seat belt restrains and protects the occupant, a large deceleration is exerted on the vehicle, so that the occupant tends to move forward due to a large inertia force. Accordingly, a large load is exerted on the seat belt so that the occupant receives a large impact load from the seat belt. Though this impact load is not so severe for the occupant, it is better to reduce the impact load.
Accordingly, a seat belt retractor which has an EA (Energy Absorbing) mechanism to restrict the load that is exerted on the seat belt in the event of an emergency when the seat belt is worn is disclosed in Japanese Utility Model Publication (KOKAI) No. S61-11085.
The seat belt retractor having the EA mechanism disclosed in this publication is provided with a torsion bar. While the seat belt is prevented from being withdrawn in the event of the emergency as mentioned above, a tensile force is exerted on the seat belt by the inertia movement of the occupant in the forward direction, so that a seat belt winding shaft rotates with twisting a torsion portion of the torsion bar, thereby withdrawing the seat belt and thus absorbing the impact energy to the occupant. After the torsion bar is twisted for a predetermined amount, the rotation of the seat belt winding shaft is stopped so as to stop the seat belt from being withdrawn, to thereby protect the occupant. The twisting of the torsion bar is also stopped so as to prevent the torsion bar from being broken.
By the way, the torsion bar in such a seat belt retractor is structured to have a uniform hardness over the entire cross section of the torsion portion or higher hardness at the outer portion than that at the center portion in the cross section of the torsion portion. As the hardness is set as mentioned above, since high stress is applied to the surface of the torsion portion when twisted, the torsion bar may be broken with a relatively small number of turns due to defect or the like on the surface. In the conventional torsion bar, therefore, the seat belt winding shaft may have a small rotational amount until the torsion portion is broken. It is difficult to ensure the enough belt withdrawing amount because of the small maximum allowable twisting deformation at which the torsion portion is not broken. When the belt withdrawing amount can not be ensured enough, there is a limitation in increasing the energy absorbing capacity.
To increase the energy absorbing capacity to the torsion bar, strict quality control for preventing defect on the surface of the torsion bar and design for making the torsion bar itself to be longer can be considered. However, the strict quality control for the defect on the surface makes the manufacturing complex, and the design for making the torsion bar to be longer makes the torsion bar to have a large size and increase the cost.
The present invention has been made under the aforementioned circumstances, and the object of the present invention is to provide a torsion bar in a seat belt retractor, wherein an energy absorbing capacity is improved without increasing the size and can be manufactured at a low cost.
To solve the aforementioned problems, the present invention provides a torsion bar in a seat belt retractor comprising at least a reel for winding up a seat belt, and a locking mechanism for preventing rotation of the reel in the belt unwinding direction when required. The torsion bar rotationally connects the reel and the locking mechanism, and has a torsion portion which is twisted when the rotation of the reel in the belt unwinding direction is stopped by the locking mechanism so as to absorb energy. The hardness at the outer portion of the torsion portion is set to be lower than the hardness at the center portion of the torsion portion.
In the torsion bar in the seat belt retractor of the present invention structured as described above, even if higher stress is applied to the outer portion of the torsion portion when the torsion bar is twisted, because the hardness at the outer portion is set to be lower than that at the center portion, the stress at the outer portion can be restrained lower than the conventional one. Therefore, the number of turns until the torsion bar is broken can be increased as compared to the conventional one.
Accordingly, the rotational amount of the reel after the twisting of the torsion portion is started can be increased as compared to the conventional one. Therefore, the belt withdrawing amount of the seat belt after the twisting of the torsion portion is started is ensured enough, thereby further improving the energy absorbing capacity.
Because the torsion bar of the present invention has improved energy absorbing capacity, the strict quality control for preventing defect on the surface of the torsion bar is not required, thereby facilitating the manufacturing of the torsion bar. Since the torsion bar of the present invention can be designed to have substantially the same size as the conventional one, increase in cost can be restrained and increase in size of the seat belt retractor is also restrained when the torsion bar is adopted to the seat belt retractor.